Torque wrenches are used to apply a controlled amount of torque to a threaded fastener. Torque wrenches which produce an indication when a pre-determined torque is reached are disclosed in, for example, U.S. Pat. No. 2,250,941 to Zimmerman, U.S. Pat. No. 3,967,513 to Myrdal, U.S. Pat. No. 4,244,434 to Wilson, U.S. Pat. No. 4,314,490 to Stone, U.S. Pat. No. 4,535,659 To Yang, U.S. Pat. No. 4,664,001 to Denman, U.S. Pat. No. 4,958,541 to Annis et al., U.S. Pat. No. 5,130,700 to Annis et al., U.S. Pat. No. 5,172,616 to Negishi and U.S. Pat. No. 6,070,506 to Becker.
U.S. Patent application Publication No. 2003/0079578A1 to Hu discloses a simplified wrench having an engaging member, mounted in a casing, and biased to press against a first section of a substantially L-shaped rod, a second section of the L-shaped rod being a driving section with means for engaging a fastener. As the engaging member presses the first section of the rod, it exerts an engaging force between the first section of the rod and the engaging member. When a rotational force applied to the casing is greater than the engaging force, the casing slides, while the rod is not turned. Such mechanisms are known as “break-away” mechanisms.
The wrench described above is purely mechanical. Indication that a pre-determined torque limit is reached is achieved by a sudden, mechanical release (“break-away”) of the torque applied to the fastener. Additionally, the wrench requires three precision mechanical parts, the first section of the rod, a spring, and a ball. Adjustment of the torque limit, or the “break-away” point, requires dismantling the apparatus and changing a component, i.e., the spring.